Field of the Invention
The invention relates to a secondary lithium-ion cell which includes at least one lithium-intercalating, carbon-containing negative electrode, a nonaqueous lithium ion-conducting electrolyte and at least one lithium-intercalating positive electrode comprising a lithium-containing chalcogen compound of a transition metal, the electrodes being separated from one another by separators.
Description of the Related Art
Lithium-ion cells have a negative and a positive electrode into each of which lithium can be reversibly intercalated. As the lithium-ion cells are charged and discharged, lithium is alternately stored in the positive and in the negative electrode, so that the total amount of cyclable charge corresponds to the amount of lithium flowing back and forth between the two electrodes. Usually, an amount of active material of one of the two electrodes is selected in assembling the lithium-ion cells such that it contains, in stored form, the amount of lithium available for subsequent cyclic operation. In many cases, the active material used as positive electrode material is a lithium-containing compound such as, but not limited to, lithium manganese spinel, lithium cobalt oxide, lithium nickel oxide or substances derived from these compounds or mixtures of these substances.
Examples of negative active materials used include carbon or metal oxides.
When the lithium-ion cells are assembled and filled with electrolyte, the active materials have a difference in potential of at most about 2 volts when compared to each other. The difference in potential between the electrodes, after the lithium-ion cell has been charged, is about 4 volts. When the lithium-ion cell is charged for the first time, lithium is deintercalated from the positive electrode and introduced into the negative electrode. As a result, the potential of the negative electrode is lowered significantly (toward the potential of metallic lithium), and the potential of the positive electrode is further increased (to even more positive levels).
These changes in potential may give rise to parasitic reactions, both on the positive and, in particular, on the negative electrode. On the surfaces of carbon negative electrodes, for example, decomposition products are known to form, which comprise lithium and components of the electrolyte (solid electrolyte interface, SEI). These surface layers, also referred to as covering layers, are lithium-ion conductors which establish an electronic connection between the negative electrode and the electrolyte and prevent the reactions from proceeding any further.
Formation of this covering layer is therefore necessary for the stability of the half-cell system comprising the negative electrode and the electrolyte. On the other hand, however, as the layer is formed, a portion of the lithium introduced into the cells via the positive electrode is irreversibly bound and thus removed from cyclic operation, i.e. from the capacity available to the user. This means that, in the course of the first discharge, not as much lithium moves from the negative electrode to the positive electrode as had previously been released to the negative electrode in the course of the first charging operation.
A further drawback is that the formation of the covering layer on the negative electrode after the first charging operation has not yet reached completion, but instead progresses further during the subsequent charging and discharge cycles. Even though this process becomes less pronounced during the further charging and discharge cycles, it still causes continuous abstraction, from the system, of lithium which is no longer available for cyclic operation and thus for the capacity of the cell.
U.S. Pat. Nos. 5,340,670 and 5,432,029 disclose active materials for negative electrodes of lithium-ion cells, which are claimed to exhibit a reduced irreversible capacity loss. Additionally, U.S. Pat. Nos. 4,980,250 and 5,436,093 disclose methods by means of which lithium is introduced into the active material of the negative electrode in order to minimize the lithium consumption and thus the irreversible capacity loss. The first-mentioned documents, however, lead to materials which still exhibit the irreversible capacity loss, and the methods for prelithiating the negative active materials lead to electrodes which can be handled only under dry-room conditions.
DE-A 195 28 049 discloses lithium-ion cells in which a lithium-rich compound such as metallic lithium or a lithium alloy has been introduced into the cells in such a way that said compound, after the cell has been filled with electrolyte, is in electrolytic contact with at least one of the electrodes. As a result of the difference in potential between the electrode materials, an equalizing current will flow between the particular active electrode material and the lithium-rich compound introduced and, as a result, additional lithium will be introduced into the electrodes. This additional lithium reduces the irreversible capacity loss arising from the abovementioned parasitic reactions or even to largely eliminate it.
A problem with this procedure, however, is that local lithium enrichment cannot be precluded. These local enrichments either represent a massive safety risk, because of the high reactivity of electrode positioned lithium metal, or they result in the lithium-storing positive electrode material being transmuted into a phase which is less suitable for further cyclic operation.
It is an object of the invention to specify a lithium-ion cell and a method for operating it, which compensate for or minimize the irreversible capacity loss and which do not result in substantial safety risks.
A preferred aspect of the invention is a secondary lithium-ion cell including a secondary lithium-ion cell comprising at least one lithium-intercalating, carbon-containing negative electrode; a nonaqueous lithium ion-conducting electrolyte; at least one lithium-intercalating positive electrode comprising a lithium-containing chalcogen compound of a transition metal; a separator separating the positive and negative electrodes; and a lithium-containing auxiliary electrode disposed within the cell, such that the auxiliary electrode is spatially separated from and positioned for selective contact, subsequent to sealing the cell, with the electrolyte, for supplying additional lithium to the cell.